Present space-based earth observation is generally from satellites in low earth orbit. This low earth orbit observation is usually accomplished with reflective telescopes. Since the position of such a low earth orbit satellite is continually changing with respect to any location on the earth's surface, any area of the earth's surface can only be viewed by the satellite for a brief time as the satellite passes over the particular area of interest on its orbit. Furthermore, if the area of interest on the earth's surface does not come within the field of regard of the satellite within an acceptable period of time, the satellite must have the capability of substantially modifying its orbit to pass over the area of interest if the desired observation is to be obtained.
A telescope in geosynchronous earth orbit, in contrast, can observe any position within its field of regard whenever desired and for as long as necessary. However, geosynchronous earth orbit is 100 times higher than low earth orbit so that to get the same resolution from geosynchronous earth orbit as from low earth orbit, the aperture of the telescope needs to be 100 times greater. Sub-meter earth observation from geosynchronous earth orbit and high resolution astronomy require space telescopes having apertures in the 10's of meters. It is apparent that a space telescope having such a large aperture would be very advantageous.
In the past, considerable effort has been spent attempting to design reflective telescopes of such size, but two basic difficulties have arisen: achieving and maintaining sub-wavelength tolerances over the large apertures, and designing telescopes which are light and compactly packaged for launch and eventual deployment into orbit. The telescope must be launchable (i.e., light weight and folded-up at launch) yet deploy to optical precision tolerances (fractional wavelengths). This has not yet been accomplished.
One prior concept concentrated on optical precision by using rigid one-to-three meter mirror segments in an aperture array wherein launchability concerns lead to the design of unfoldable segmented sparse aperture arrays. Another concept concentrated on reducing mass and improving deployability by employing a thin membrane mirror. However, optical precision concerns demand the presence of high frequency (space and time) adaptive optics.